Digital to analog converters hereinafter referred to as (DACs) are typically a plurality of current sources that can be set to determine a specified current at the output of the DAC. Resistor trimming is used to improve the accuracy and monotonicity of such DAC's.
Well controlled, fixed reference currents are often provided in electrical circuits by actively trimming a plurality of resistor values until the desired current level is obtained. A typical way of accomplishing this is taught, for example in U.S. Pat. No. 4,799,043, wherein there is disclosed a circuit resistance adjusting device for producing a combined resistance consisting of a plurality of resistances which are selectively connected by switching members containing fuses. Selected resistors are switched into the circuit by blowing a fuse to set a desired resistance level in the circuit.
In this patent however, the target current is permanently set, i.e., the connection of the resistors is fixed by the fuses, and once set is not restorable.
The present invention overcomes the difficulty of having fixed resistors by a circuit in which fuses may be set or blown to set a fixed target current in the device yet will permit the DAC output current to remain alterable if conditions require such an alteration.
The selectable control approach of the present invention particularly provides a low cost solution for such later altering. Also, because the circuit can be selectively programmed at different current levels it is useful in overall development design by providing a reduction in the overall time needed to develop a particular device as well as providing the means wherein the digital interface can still be utilized to later adjust the output if the initial setting proves inadequate.
Thus, the present invention permits field repair as resistance changes with time or if a resistor fails.